This invention relates to plate fin and tube heat exchangers used in air conditioning, refrigeration and other applications. More particularly, the invention relates to an improved plate fin for use in a heat exchanger in which condensation of a vapor entrained in the gaseous fluid flowing over the exterior of the tubes and plate fins is likely.
Plate fin and tube heat exchangers are used in a wide variety of applications in which it is desired to exchange heat between two fluids, usually a pure liquid or a liquid undergoing a phase change to or from a gas, flowing in the heat exchanger tubes and a gas, usually air, flowing around the heat exchanger plate fins and tube exteriors. In such a heat exchanger, a plurality of thin plate fins are arranged parallel to each other between two tube sheets. Heat exchanger tubes pass through holes in the tube sheets and plate fins. There is a firm fit between the tubes and the plate fins so that the effective surface area, and thus the heat transfer area, of the heat exchanger tubes is increased by the area of the plate fins. Because of this increase in surface area, a plate fin and tube heat exchanger offers improved heat transfer performance over a plain tube type heat exchanger of the same size.
Prior art designers have devised numerous plate fin configurations. The configurations developed have attempted to improve the heat transfer performance of a given plate fin in two primary ways: (1) by maximizing, within the limits of the heat exchanger external dimensions, the plate fin surface area in contact with the fluid flowing around the fins; and (2) by configuring the fin in such a way as to minimize the thickness of the heat transfer inhibiting boundary layer on the external surfaces of the fin. One means of increasing the fin surface area is to corrugate the fin so that, for a given fin spacing, more fin surface area can be fit into the same volume. Corrugation also promotes conditions that minimize boundary layer thickness.
Another means of minimizing boundary layer thickness is to configure the fins with louvers or lances. A louver is a raised portion of the fin formed by first making a single slit into the fin and then raising the fin material on one side of the slit. A lance is a raised portion of the fin formed by first making two slits into the fin and then raising the fin material between the slits.
U.S. Pat. No. 4,860,822 (Sacks, Aug. 29, 1989) describes a heat exchanger plate fin that incorporates more than one type of heat transfer performance enhancement. The Sacks fin is corrugated in a sinusoidal pattern, with raised lance elements formed into the surface of the fin.
In some applications, there may be a gaseous fluid having an entrained vapor flowing around the tubes and plate fins and conditions in the heat exchanger may be conducive to condensation of the vapor. An example of such an application is the evaporator in an air conditioning system. Indeed, one function of an air conditioning evaporator is to condense moisture from the air passing through it. A heat exchanger designed for use in an application where vapor condensation may occur must provide for effective and efficient drainage of condensate from the tubes and plate fins. If condensate cannot quickly drain off, it will build up on and blanket the exterior surfaces of the tubes and fins, thus reducing heat transfer and also increasing the air flow resistance of the heat exchanger.
Surface enhancements on a plate fin such as louvers and lances may inhibit drainage of condensate from the fin, as surface tension effects may cause condensate droplets to bridge between the edges of the louver or lance and the main body of the fin, causing surface blanketing and interfering with the function of the surface enhancement.
Thus a plate fin designed for use in an environment where the fluid flowing over the tubes and fins is completely gaseous may not perform well in an environment in which condensation of entrained vapor occurs on the fin.